8.1.1.1
This section presents the risk
assessment of landfill gas (LFG) hazards arising from the construction and
operation phases of the Project with reference to Section 3.4.9 the EIA Study
Brief.
8.2.1
General
8.2.1.1
Under Annexes 7 and 19 of the
Technical Memorandum of the Environmental Impact Assessment Ordinance
(EIAO-TM), landfill gas (LFG) hazard assessment is required for any development
or re-development within the Consultation Zone. The Project falls within the
Consultation Zone (CZ) of Gin Drinkers Bay Landfill (GDBL). Therefore, LFG hazard assessment is required for
the Project.
8.2.1.2
Environmental Protection
Department (EPD) has issued two guidance notes regarding landfill gas hazard
assessment as an assessment framework to be followed when evaluating the risks
related to developments described under Section 6.5, Chapter 9 of the Hong Kong
Planning Standards and Guidelines:
·
ProPECC PN 3/96 – Landfill Gas Hazard Assessment for Development Adjacent
to Landfill; and
·
EPD/TR8/97 – Landfill Gas
Hazard Assessment Guidance Note.
8.2.1.3
It is a requirement that
project proponents of developments adjacent to landfills undertake a landfill
gas hazard assessment and submit the findings to EPD for vetting. As recommended in ProPECC
PN 3/96, the project proponent and professionals (Authorised Persons)
responsible for the developments adjacent to landfills should:
·
carry out a landfill gas hazard
assessment to evaluate the degree of risk associated with the proposed development;
·
design suitable
precautionary/protection measures to render the proposed development as safe as
reasonably practicable;
·
ensure that the
precautionary/protection measures will be implemented and constructed in
accordance with the design; and
·
establish a maintenance and
monitoring programme for ensuring the continued performance of the implemented
protection measures.
8.3.1
Approach
8.3.1.1
In accordance with the Landfill
Gas Hazard Assessment Guidance Note, the risk due to landfill gas may be
evaluated based upon the following three criteria:
·
Source – location, nature and
likely quantities / concentrations of landfill gas which has the potential to
affect the development;
·
Pathway – the ground and
groundwater conditions, through which landfill gas must pass in
order to reach the development; and
·
Target – elements of the
development that are sensitive to the effects of landfill gas.
8.3.2.1
The classification of the
Source (i.e. the landfill) should be undertaken as
follows:
Table 8.1 Classification of the Source
|
Minor
|
Landfill sites at which gas controls have
been installed and proven to be effective by comprehensive monitoring which
has demonstrated that there is no migration of gas beyond the landfill boundary
(or any specific control measures) and at which
control of gas does not rely solely on an active gas extraction system or any
other single control measure which is vulnerable to failure; or
Old landfill sites where the maximum
concentration of methane within the waste, as measured at several locations
across the landfill and on at least four occasions over a period of at least
3 months (preferably longer), is less than 5% by volume (v/v).
|
|
Medium
|
Landfill site at which some form of gas
control has been installed (e.g. lined site or one
where vents or barriers have been retrospectively installed) but where there
are only limited monitoring data to demonstrate its efficacy to prevent
migration of gas; or
Landfill site where comprehensive
monitoring has demonstrated no significant migration of gas beyond the
landfill boundary but where the control of gas relies solely on an active gas
extraction system or any other single control system which is vulnerable to failure.
|
|
Major
|
Recently filled landfill site at which there
is little or no control to prevent migration of gas or at which the efficacy
of the gas control measures has not been assessed; or
Any landfill site at which monitoring has
demonstrated that there is significant migration of gas beyond the site boundary.
|
8.3.2.2
The 'significance' of migration
should be assessed by reference to the concentration, frequency and location at
which gas is detected. For guidance, it
should be assumed that any concentration of methane or carbon dioxide greater
than 5% v/v above background levels in any monitoring well outside the
landfill's boundary indicates significant migration. Lower concentrations may still be
'significant' if they are observed in more than one monitoring well, on several
occasions or in monitoring wells located some distance from the site
boundary. In general, concentrations of
greater than 1% v/v methane or 1.5% v/v carbon dioxide (above background levels
in each case) indicate less than adequate control of the gas at source.
8.3.2.3
In classifying the source,
account needs to be taken of the likelihood and probable effect of a failure of
the gas controls. Thus, if it has been demonstrated that there is no migration
of gas and there is little danger of the gas controls failing (e.g. if these comprise solely of passive measures such as a
liner) it can be assumed that the site represents a "Minor"
Source. Where there is no gas migration,
but this may be as a result of a single, "vulnerable" control measure
(e.g. an active extraction system with no warning of failure),
the site should be regarded as a "Medium" or even a "Major"
Source depending on the other factors (e.g. size of site and age of waste).
8.3.2.4
Where the effectiveness of the
gas controls has not been proven by off-site monitoring or if there is some
doubt as to the adequacy of the monitoring, this should be taken
into account when considering the impact of the control measures on the
Source term.
8.3.2.5
Assessments should always err
on the side of caution and, in general, if the effectiveness cannot be demonstrated,
the assessment should be undertaken on the same basis as if the controls were
not in place.
8.3.2.6
The reliability of the
monitoring, for determining the efficacy of the gas controls, needs to take
account of the design, number and location of the
monitoring points together with the frequency and duration over which
monitoring has been undertaken.
Monitoring should have been undertaken under different weather
conditions including, in particular, periods of low or
falling atmospheric pressure.
8.3.3
Pathway
8.3.3.1
The broad classification of the
Pathway should be undertaken as follows
Table 8.2 Classification of Pathway
|
Very short / direct
|
Path length of
less than 50m for unsaturated permeable strata and fissured rock or less than
100m of man-made conduits
|
|
Moderately short / direct
|
Path length of
50~100m for unsaturated permeable soil or fissured rock or 100~250m for
man-made conduits
|
|
Long / indirect
|
Path length of
100~250m for unsaturated permeable soils and fissured rock
|
8.3.3.2
In classifying the pathway,
however, adjustment to the above general guidelines will often be required to
take account of other factors which will affect the extent of gas migration
including the following:
·
Particular permeability of the soils;
·
Spacing, tightness and
direction of the fissures/joints;
·
Topography;
·
Depth and thickness of the
medium through which the gas may migrate (which may be affected by groundwater
level);
·
The nature of the strata over
the potential pathway;
·
The number of different media
involved; and
·
Depth to groundwater table and
flow patterns.
8.3.3.3
Thus, although there may be
permeable soil between the landfill site and a proposed development, if the
soil layer is very shallow and thin with its upper surface exposed to the
atmosphere, then it will be appropriate to consider this as a long/indirect
pathway. This could alter if the land
between the landfill site and the development was altered in some other way
which reduced the potential for gas release.
Similarly, if the land is flat, the surface may be prone to water
logging which will also effectively seal it at times of heavy rain. In general, a conservative approach should be
adopted, and it should be assumed that any such permeable surface soils may
become less permeable in the future.
8.3.3.4
If it is known that a conduit
(man-made or natural feature such as a fault plane) leads directly from the
landfill to the development area, it should be regarded as a
"direct/short" pathway even if it is longer than 100m.
8.3.4
Target
8.3.4.1
Different types of target may be broadly classified as follows:
Table 8.3 Classification of Target
|
High sensitivity
|
Buildings and structures with ground
level or below ground rooms/voids or into which services enter directly from
the ground and to which members of the general public
have unrestricted access or which contain sources of ignition. This would include any developments where
there is a possibility of additional structures being erected directly on the
ground on an ad hoc basis and thereby without due regard to the potential
risks.
|
|
Medium sensitivity
|
Other buildings, structures
or service voids where there is access only by authorized, well trained
personnel, such as the staff of utility companies, who have been briefed on
the potential hazards relating to landfill gas and the specific safety
procedures to be followed.
Deep excavations.
|
|
Low sensitivity
|
Buildings/structures which are less prone
to gas ingress by virtue of their design (such as those with a raised floor
slab). Shallow excavations. Developments which involve essentially
outdoor activities but where evolution of gas could pose potential problems.
|
8.3.5
Risk Categorization
8.3.5.1
The classification of the above
LFG sources, pathway and target are categorized. Having determined which categories of source,
pathway, target and the various elements of the development fall, overall
assessment of risk may be made.
8.3.5.2
Table 8.4
presents classification of risk categories whilst potential implications
associated with the various qualitative risk categories are summarized in Table 8.5.
Table 8.4 Classification of Risk Category
|
Source
|
Pathway
|
Target
Sensitivity
|
Risk
Category
|
|
|
|
High
|
Very High
|
|
|
Very short / direct
|
Medium
|
High
|
|
|
|
Low
|
Medium
|
|
|
|
High
|
High
|
|
Major
|
Moderately short / direct
|
Medium
|
Medium
|
|
|
|
Low
|
Low
|
|
|
|
High
|
High
|
|
|
Long / indirect
|
Medium
|
Medium
|
|
|
|
Low
|
Low
|
|
|
|
High
|
High
|
|
|
Very short / direct
|
Medium
|
Medium
|
|
|
|
Low
|
Low
|
|
|
|
High
|
High
|
|
Medium
|
Moderately short / direct
|
Medium
|
Medium
|
|
|
|
Low
|
Low
|
|
|
|
High
|
Medium
|
|
|
Long / indirect
|
Medium
|
Low
|
|
|
|
Low
|
Very Low
|
|
|
|
High
|
High
|
|
|
Very short / direct
|
Medium
|
Medium
|
|
|
|
Low
|
Low
|
|
|
|
High
|
Medium
|
|
Minor
|
Moderately short / direct
|
Medium
|
Low
|
|
|
|
Low
|
Very Low
|
|
|
|
High
|
Medium
|
|
|
Long / indirect
|
Medium
|
Low
|
|
|
|
Low
|
Very Low
|
Table 8.5 Summary of General Categorization of Risk
|
Category
|
Level of Risk
|
Implication
|
|
A
|
Very High
|
The type of development being proposed
is undesirable and a less sensitive form of development should be considered.
At the very least, extensive
engineering measures, alarm systems and emergency action plans are likely to be
required.
|
|
B
|
High
|
Significant engineering measures will
be required to protect the planned development.
|
|
C
|
Medium
|
Engineering measures will be required
to protect the proposed development.
|
|
D
|
Low
|
Some precautionary measures will be
required to ensure that the planned development is safe
|
|
E
|
Very Low (insignificant)
|
The risk is so low that no
precautionary measures are required.
|
8.3.5.3
Five generic forms of
protection will be used in mitigating the hazards to development. These generic forms corresponding to the five
risk levels are set out in Table 8.6.
The terms used in Table 8.6 are
defined in Table 8.7.
Table 8.6 Generic Protection Measures for Planning Stage Categorization
|
Category
|
Implication
|
|
A
|
For the planned development active
control of gas, supported by barriers and detection systems. Another, less sensitive
form of development should also be considered.
|
|
B
|
Active control of gas, including
barriers and detection systems (1).
|
|
C
|
Use of “semi active” or enhanced
passive controls. Detection systems in
some situations.
|
|
D
|
Passive Control of gas only.
|
|
E
|
No precautionary measures required.
|
Note
(1): The gas protection measures
required to allow the safe development of a Category A risk development will
need to be more extensive than those for a Category B risk development.
Table 8.7 Definition of Control Terms
|
Terms
|
Definition
|
|
Active control
|
Control of gas by mechanical means e.g. ventilation of spaces with air to dilute gas, or
extraction of gas from the development site using fans or blowers.
|
|
“Semi active” control
|
Use of wind driven cowls and other
devices which assist in the ventilation of gas but do not rely on
electrically powered fans.
|
|
Passive control
|
Provision of barriers to the movement
of gas e.g. membranes in floors or walls, or in
trenches, coupled with high permeability vents such as no-fines gravel in
trenches or voids/permeable layers below structures.
|
|
Detection systems
|
Electronic systems based upon, for example,
catalytic oxidation or infra-red measurement principles, which can detect low
concentrations of gas in the atmosphere and can be linked to alarms and/or
telemetry systems.
|
8.4.1
History of Gin Drinker Bay
Landfill
8.4.1.1
Prior to 1960, Gin Drinkers Bay
was open water between Pillar Island and Kowloon with a maximum water depth of
about 13m. Before tipping commenced, a
rock bund was built connecting the island to the mainland. Waste was tipped behind the rock bund. Tipping into open water ceased in 1967
although "open tipping" continued until 1973 when controlled
landfilling was adopted. Waste deposition ceased in 1979. It is estimated that between 8 and 12 million
tonnes of domestic and industrial wastes were deposited. The thickness of the
waste increases from the perimeter towards the centre platform at about +45mPD.
Under the platform, the base of the waste is approximately -10mPD, giving a
maximum waste depth of approximately 55-60 m deep. At its deepest part, waste extends 10-14 m
below sea level. The site was not lined
prior to filling although a perimeter leachate collection system was installed
along the toe of the fill that discharges by gravity into the foul sewer
system. The cover material over the
waste comprises silty sand and gravel derived from completely decomposed
granite (CDG) with additional clayey fill in some locations.
8.4.1.2
Passive LFG vents were
installed around the perimeter but there were no provisions for the collection
of leachate, which seeped out of the base of the
landfill and into the adjacent Rambler Channel. When leachate levels were
elevated, leachate often overflowed onto low-lying parts of the roads and
footpaths surrounding the site.
8.4.1.3
Following completion of the
filling and placement of the cover soils, several thousand trees were planted
on the site and preliminary works for development of the site as a community
park (Kwai Chung Park) were undertaken by Regional Services Department. These
works were suspended in the early 1990s as plans were finalised for the
construction of a MTRC viaduct that cut through part of the site. The portion
where MTRC's Lantau and Airport Railway cut through the waste have been capped
with a polyethylene membrane and restoration soil. The intention was to control the infiltration
of rainwater and the release of landfill gas.
8.4.1.4
In common with the other closed
landfill sites in Hong Kong, restoration works were deemed necessary to reduce
the potential health and environmental risks associated with LFG and landfill
leachate, and to allow beneficial afteruse of the
site. The restoration works for GDBL were carried out as part of a
Design-Build-Operate (DBO) contract led by Government, under the management of
EPD. Restoration works commenced in February 1999 and were completed in
September 2000.
8.4.1.5
The Restoration Contractor
installed an active LFG collection system, including gas headers, gas
extraction wells, a flare and an electricity
generator. A new leachate collection system was installed, as controlling
leachate levels was considered to be of paramount
importance because of its detrimental effects on the MTRC viaduct. A Leachate
Treatment Plant (LTP) was constructed on a platform below the MTRC viaduct to
treat leachate from the landfill. A new geosynthetic capping system was
installed on top of the platforms, with the heavily vegetated side slopes left
uncapped. Prior to landscaping, a number of
groundwater, LFG and leachate monitoring wells were installed.
8.4.1.6
As part of the restoration
works of the GDBL under Contract No. EP/SP/30/95 North West
New Territories Landfills and Gin Drinkers Bay Landfill Restoration let by the
EPD, a horizontal perimeter trench with an active gas collection system was
constructed around the entire site to extract and flare or utilise the landfill
gas. A new geosynthetic capping system comprising a linear low
density polyethylene (LLDPE) geomembrane, geocomposite
drainage layer and a 850 to 1500 mm thick general cover layer of completely
decomposed granite / completely decomposed volcanic tuff (CDG/CDV) was
installed on top of the platforms however heavily vegetated side slopes were
left uncapped. Prior to landscaping, a number of
groundwater, LFG and leachate monitoring wells were installed.
8.4.1.7
The design for the LFG
management system prepared by the Landfill Restoration Contractor includes
active landfill gas management to extract and treat LFG and the restoration
capping system. The system is less prone
to failure due to automated controls and is integrated with the leachate
management system with a backup power supply and workshop for rectification/
maintenance. The gas treatment facility is situated to the centre of the
landfill. Under the performance requirements for the Landfill Restoration
Contract, the treatment process has been designed to prevent the emission of
unacceptable levels of methane, carbon dioxide, odourants
and volatile organics to the atmosphere. The Landfill Restoration Contractor is
obliged to ensure that no adverse environmental impacts arise from the landfill
gas treatment process either off-site or on site.
8.4.1.8
A number of gas monitoring wells have been installed around the site to monitor
potential off-site landfill gas migration.
The location of the monitoring wells within the landfill and within the
vicinity of the Project are presented in Appendix 8.1 and Figure 8.1.
8.4.2
Recent Monitoring Data of Gin
Drinkers Bay Landfill
8.4.2.1
Five (5) monitoring locations
are situated adjacent west of the by-pass alignment (GDB5, GDBGW7, GDB7, GDB10
and GG3) and five (5) monitoring locations are situated to the east of the
alignment (G1, SVGDB6, SV23B, GDB6 and SV24B).
Relevant wells within the vicinity of the alignment to the north of the
landfill include GDB4 and SVGDB4. These
wells are selected as the nearest to the by-pass alignment adjacent to GDBL,
therefore most relevant to the assess landfill gas hazard to the Project.
8.4.2.2
Concentrations of methane and
carbon dioxide present in LFG are monitored in the aforementioned
monitoring wells on a monthly basis as part of the North-West New
Territories Landfills and Gin Drinkers Bay Landfill Restoration Contract No.
EP/SP/30/95.
8.4.2.3
Monthly landfill gas monitoring
data from July 2020 to June 2022 provided by Hong Kong Environmental Protection
Department is summarized in Table 8.8 with
complete records present in Appendix
8.2.
Table 8.8 Gas Monitoring Data for GBD Landfill (July 2020 ~ June 2022)
|
Monitoring Location
|
Average Methane
% v/v
(Range)
#
|
Average Carbon
Dioxide % v/v
(Range)
#
|
|
GDB5
|
All
measurements below detection*
|
0.0 (<0.1 -
0.1)
|
|
GDB7
|
0.0 (<0.1 – 0.1)
|
7.7 (1.5 -
13.8)
|
|
GDB10
|
0.0 (<0.1 – 0.1)
|
4.6 (<0.1 -
13.2)
|
|
GDBGW7
|
All
measurements below detection*
|
0.1 (<0.1 -
0.9)
|
|
GG3
|
All
measurements below detection*
|
4.2 (2.6 -
5.9)
|
|
|
|
|
|
G1
|
All
measurements below detection*
|
6.0 (1 - 13.2)
|
|
GDB6
|
All
measurements below detection*
|
2.2 (<0.1 -
5.3)
|
|
SV23B
|
All
measurements below detection*
|
0.0 (<0.1 -
0.2)
|
|
SV24B
|
All
measurements below detection*
|
All
measurements below detection*
|
|
SVGDB6
|
0.0 (<0.1 –
0.2)
|
0.0 (<0.1 -
0.3)
|
|
|
|
|
|
GDB4
|
All
measurements below detection*
|
0.3 (<0.1 -
1.7)
|
|
SVGDB4
|
All measurements
below detection*
|
All
measurements below detection*
|
Note: * the detection limit is 0.1% v/v.
# zero value is assigned to measurement for
below detection limit when
averaging.
8.4.2.4
Leachate quality information
has not been requested as the major problem associated with leachate is high
concentrations of ammonia and organic nitrogen which may lead to eutrophication
of aquatic environments rather than production of landfill gas under anaerobic
conditions.
8.4.3
Landfill Gas Source Summary
8.4.3.1
GDBL is a restored gassing
landfill with gas control measures consisting both
active and passive landfill gas management systems. Active control of gas is vulnerable to system
failure however GDBL infrastructure includes backup power generation and a
maintenance facility which suggest that system failure is unlikely to result in
short-term uncontrolled release of landfill gas.
8.4.3.2
Comprehensive monitoring data
from wells outside the landfill confirms no migration of methane beyond GDBL,
however some elevated carbon dioxide / depleted oxygen concentrations are
detected. In accordance with
EPD’s Guidance Note, levels of carbon dioxide that exceed 5% above
background would be considered “significant” to the extent that the potential
for off-site migration of landfill gas cannot be eliminated, however, as no
methane is detected and as carbon dioxide may be present in soil gas as a
result of the oxidation of organic matter from source such as leakage from foul
sewers, organic sediments or possible pockets of waste deposited beyond the
known landfill boundary when tipping of waste was not controlled; the
likelihood of migration of landfill gas from the restored GDBL is considered
low. Since possible pockets of waste may have deposited beyond the known
landfill boundary and the excavation was in close proximity
to the landfill boundary, potential encounter on waste pockets during
excavation could occur. No major excavation is anticipated and such excavation which
would not encroach into / penetrate the capping layer of the landfill,
therefore, landfill restoration facilities will not be affected. The Source is conservatively classified as Medium.
8.4.4.1
The potential pathways by which
LFG may migrate fall into three categories: (i) man-made pathways including
service ducting, tunnels and culverts, (ii) pathways
including porous soil, planar openings such as joints, and geological faults
and (iii) a combination of natural and man-made pathways.
8.4.4.2
No utility runs or fault lines
are mapped between the landfill and adjacent section of Tsuen Wan Road. The
waste is underlain by marine deposits and alluvial / colluvial sediments, that
are in turn underlain by varying thicknesses of moderately or completely
decomposed granite bedrock. Marine deposits are typically of low gas
permeability whilst unconsolidated granular soils such as alluvial / colluvial
deposits and decomposed granite may exhibit higher intrinsic permeability
potentially allowing passage of gas via pressure gradients. The landfill is
capped thus reducing vertical gas transport pathways to atmosphere.
8.4.4.3
Whilst no utility runs or fault
lines are mapped between the landfill and adjacent section of Tsuen Wan Road,
as the development is directly adjacent to the GDBL, a conservative path length
of less than 50m is assumed for unsaturated permeable strata, therefore the
pathway is classified as Very Short/ Direct for the construction phase
works at ground level or below ground level and for operation phase.
8.4.5.1
The road scheme will be
developed in a primarily outdoor setting where evolution of gas is unlikely to
pose any potential problems. Construction will be undertaken by workers who are
well trained and where safe construction methodologies are employed. Deep excavation for construction of road
piers may require hot works, however risk can be mitigated by adopting standard
protection measures. The construction
Phase target classification is considered to be Low
Sensitivity.
8.4.5.2
For road user of Tsuen Wan road, no indoor activity is anticipated and they
are considered as Low Sensitivity.
8.5.1.1
In accordance with the Landfill
Gas Hazard Assessment Guidance Note, risk due to landfill gas has been
qualitatively evaluated based upon a description and assessment of
relationships between the source of landfill gas, realistic gas migration
pathways and an evaluation of the sensitivity of construction and operation
phase targets.
8.5.1.2
Table 8.9
summarises the source and pathway classifications, target sensitivities and
overall risk categorization.
Table 8.9 Overall Risk Categorization
|
Source
|
Pathway
|
Target
Sensitivity
|
Risk
Category
|
|
Restored Gin Drinkers Bay Landfill
Medium Source
|
Very Short/Direct
(GDBL is adjacent therefore, a path length of
<50m is assumed)
|
Construction Phase
Low
Sensitivity
|
Construction Phase
Category D
Low Risk
|
|
Very Short/Direct
(GDBL is adjacent therefore, a path length of
<50m is assumed)
|
Operation
Phase
Low
Sensitivity
|
Operation Phase
Category D
Low Risk
|
8.5.1.3
Overall assessment of risk
classifies the construction phase is a “Low” risk situation, however as
the road alignment passes through the GDBL Consultation Zone, guidelines and
recommendations relating to general hazards which may be encountered during
construction and an outline of safety requirements as stated in Chapter 8 of
the Landfill Gas Hazard Assessment Guidance Note may be used to form the
basis of Specification Clauses for incorporation in Contract Documentation.
8.5.1.4
The operation phase is
classified as a “Low” risk situation. Since no indoor activity
is anticipated for road user, no specific mitigation is required.
8.5.1.5
Provided that the safety requirements
stated in Chapter 8 of the Landfill Gas Hazard Assessment Guidance Note are
implemented properly, the safety of the site workers and future users
thereafter would be safeguarded, and no adverse landfill gas hazard is
anticipated.
8.6.1
Construction Phase
8.6.1.2
The monitoring requirement as stated
in Chapter 8 of the Landfill Gas Hazard Assessment Guidance Note has been
detailed below:
8.6.1.3
Monthly gas monitoring should
also be conducted for offices, stores etc (if any) set up within areas of the
Project location with GDBL and its 250m Consultation Zone. Monitoring
requirements and procedures specified in Paragraphs 8.23 to 8.28
of EPD’s Guidance Note are highlighted as follows:
·
The monitoring equipment used
should be capable of measuring methane, carbon dioxide and oxygen concentrations.
The equipment should be intrinsically safe and calibrated according to the
manufacturer’s instructions.
·
When portable monitoring
equipment is to be used, the frequency and areas to be monitored should be set
down prior to commencement of the works either by the Safety Officer or by an
appropriately qualified person.
·
All measurements should be made
with the monitoring tube located not more than 10 mm from the surface.
·
A standard form, detailing the
location, time of monitoring and equipment used together with the gas
concentrations measured, should be used when undertaking manual monitoring to
ensure that all relevant data are recorded.
·
If methane (flammable gas) or
carbon dioxide concentrations are in excess of the
trigger levels or that of oxygen is below the level specified in the Emergency
Management in the following sections, then evacuation should be initiated.
8.6.1.5
Routine monitoring should be
carried out in all excavations, manholes, chambers, relocation of monitoring
wells and any other confined spaces that may have been created. All measurements in excavations should be made
with the extended monitoring tube located not more than 10 mm from the exposed
ground surface. Monitoring should be
performed properly to make sure that the area is free of LFG before any man
enters into the area.
8.6.1.6
For excavations deeper than 1m,
measurements should be carried out:
·
at the ground surface before
excavation commences;
·
immediately before any worker
enters the excavation;
·
at the beginning of each
working day for the entire period the excavation remains open; and
·
periodically throughout the
working day whilst workers are in the excavation.
8.6.1.7
For excavations between 300mm
and 1m deep, measurements should be carried out:
·
directly after the excavation
has been completed; and
·
periodically whilst the
excavation remains open.
8.6.1.8
For excavations less than 300mm
deep, monitoring may be omitted, at the discretion of the Safety Officer or
other appropriately qualified person.
8.6.1.9
Depending on the results of the
measurements, actions required will vary and should be set down by the Safety
Officer or other appropriately qualified person.
8.6.1.10
As a minimum these should
encompass those actions specified in below table:
|
Parameter
|
Measurement
|
Action
|
|
Oxygen
|
< 19%
|
·
Ventilate to restore oxygen to > 19 %
|
|
|
< 18%
|
·
Stop works
·
Evacuate personnel/prohibit entry
·
Increase ventilation to restore oxygen to > 19 %
|
|
Methane
|
> 10 % LEL (i.e. > 0.5 % by volume)
|
·
Prohibit hot works
·
Ventilate to restore methane to < 10% LEL
|
|
|
> 20 % LEL (i.e. > 1 % by volume)
|
·
Stop works
·
Evacuate personnel/prohibit entry
·
Increase ventilation to restore methane to < 10 % LEL
|
|
Carbon Dioxide
|
> 0.5 %
|
·
Ventilate to restore carbon dioxide to < 0.5%
|
|
|
> 1.5 %
|
·
Stop works
·
Evacuate personnel/prohibit entry
·
Increase ventilation to restore carbon dioxide to <0.5%
|
8.6.1.11
In order to ensure that evacuation procedures are implemented in the event of
the trigger levels specified in the above table being exceeded, it is
recommended that a person, such as the Safety Officer, is nominated, with
deputies, to be responsible for dealing with any emergency which may occur due
to LFG.
8.6.1.12
In an emergency situation the
nominated person, or his deputies, shall have the necessary authority and shall
ensure that the confined space is evacuated and the
necessary works implemented for reducing the concentrations of gas.
8.6.1.13
For excavation within GDBL, due
to potential encounter on waste pockets during excavation, the following
measures should be implemented.
·
Prominent LFG safety warning
signs should be erected on-site to alert all personnel and visitors of the
hazards during excavation works. No smoking or burning should be permitted
on-site in the working area, and prominent ‘No smoking’ and ‘No Naked Flames’
sign should be erected on-site where appropriate. No worker should be allowed
to work alone at any time in excavated trenches or confined areas on-site.
·
Adequate fire
fighting equipment should be provided on-site. Construction equipment
should be equipped with a vertical exhaust at least 0.6m above ground installed
with spark arrestors. Electrical motors and extension cords should be
explosion-proof and intrinsically safe when being used on-site.
·
‘Permit to Work’ system should
be implemented in accordance with the guidance on entry into confined spaces
provided in ‘Code of Practice on Safety and Health at Work in Confined Spaces’
issued by Labour Department of HKSAR Government. Welding, flame-cutting or
other hot works should be conducted only under ‘Permit to Work’ system
following clear safety requirements, gas monitoring procedures and in the
presence of qualified persons to oversee the works.
·
For piping assembly or conduit
construction, all valves and seals should be closed immediately after
installation to avoid accumulation and migration of LFG. If installation of
large diameter pipes (diameter >600mm) is required, the pipe ends should be
sealed on one side during installation. Forced ventilation is required prior to
operation of the installed pipeline. Forced ventilation should also be required
for works inside trenches deeper than 1m.
·
The frequency and location of
LFG monitoring within the excavation area should be determined prior to
commencement of works. LFG monitoring in excavations should be conducted at no
more than 10mm from the exposed ground surface. For excavation works, LFG
monitoring should be conducted
(1) at ground
surface prior to excavation,
(2)
immediately before workers entering excavations,
(3) at the
beginning of each half-day work, and
(4)
continuously throughout the working day when workers are in the excavation.
·
Any cracks on ground level
encountered on-site should be monitored for LFG periodically. Appropriate
action should be taken in accordance with the action plan as shown in Section
8.6.1.10.
·
LFG precautionary measures
involved in excavation and piping works should be provided in accordance with
the LFG Guidance Note and included in the Safety Plan for the construction
phase of the Project.
·
Temporary offices or buildings
should be located where free LFG has been proven or raised clear of ground at a
separation distance of at least 500mm.
8.6.2
Operation Phase
8.7.1.1
Overall landfill gas hazard
assessment for the construction phase and operation phase of the Project is
categorized as “Low”.
8.7.1.2
Provided that the safety
requirements stated in Chapter 8 of the Landfill Gas Hazard Assessment Guidance
Note are implemented properly, no adverse landfill gas hazard is anticipated
during the construction phase.
8.7.1.3
No precautionary measures are
required for the operation phase of the Project.